(1) Field of the Invention
This invention relates to a PCM coder and decoder, a so-called PCM CODEC, and more particularly to a PCM CODEC which has the function that three or more subscriber parties can hold a conference when the PCM CODEC is utilized as a subscriber line interface circuit in an electronic switching system.
(2) Description of the Prior Art
In a PCM telecommunications system, an analog voice signal which is coded into a PCM signal is transmitted to the other party where the PCM signal is decoded into the original analog signal. Accordingly, a PCM CODEC is provided for both the transmitter and the receiver.
In recent years, together with the remarkable progress in semiconductor fabrication processes, PCM CODECs have been realized by LSI circuits, and thus a time-division multiplexing system, a so-called single-channel CODEC system, is mainly employed. In a multiplexing system, an LSI PCM CODEC is provided for each subscriber line in a time-division electronic switching system and a transmission system, and a signal is first converted into a PCM signal at low speed, and the PCM signal is the time-division multiplexed.
In the same way as in conventional methods, this PCM telecommunication is performed with a one-to-one relationship between receiver and transmitter, and PCM telecommunication provided by an LSI single-channel CODEC, whose main purpose is economy, can not achieve a conference of more than this one-to-one ratio, such as a three-party conference. However, it is theoretically possible to achieve a three-party conference with a conventional single-channel CODEC (for example, refer to Japanese Patent Laid-Open No. 73937/1981). The CODEC normally operates so that after the receipt of a synchronizing signal at a period of 8 KHz, a subsequent PCM signal (normally 8-bits long) is received during a predetermined time only, and is then D/A converted to obtain an analog voice signal. Thus it is possible to enable one party of a one-to-one conference to receive the voice of a third party by supplying a synchronizing signal of a coder allotted to a CODEC used by the third party to at least one decoder of the CODECs in the one-to-one conference (i.e., by supplying two synchronizing signals within the 8 KHz period).
However, in this case, in order to provide a three-party conference, it is necessary that the synchronizing signals used for decoding for the first two conference parties are simultaneously supplied to all the CODECs used by the three parties, and that a digital-to-analog converter decoding for each CODEC must operate at least twice as fast. These are conditions for a three-party conference, but when the number of parties is increased to four or more, it is necessary to increase the number of synchronizing signals supplied to each decoder for decoding, according to the number of parties, and simultaneously increase the operating speed of all the decoders. The increase in the number of supplied synchronizing signals increases the complexity of the telecommunication system, also the operating speed of the decoders, so that it is necessary to employ expensive, impractical high-speed elements. This means that in practice it has been impossible to realize a conference with an LSI single-channel CODEC system.